The surfaces of glass sheets and other substrates can be made hydrophobic or hydrophilic by forming recesses and projections on the surfaces.
In general, the wettability of a solid surface is affected by the roughness of that surface. That is, the hydrophilicity of a rough surface increases if the solid surface is hydrophilic, whereas the hydrophobicity a rough surface increases if the solid surface is hydrophobic. This phenomenon is noticeable in a surface with a fractal structure, and thus because of this property a fractal surface can become a so-called super hydrophobic or super hydrophilic surface.
It should be noted that a hydrophobic state in which the contact angle of water exceeds 150° is generally referred to as “super hydrophobic”, whereas a hydrophilic state in which even measurement of the contact angle of water is difficult is referred to as “super hydrophilic”.
For example, (1) JP H06-25449A discloses a method of forming minute projections by subjecting the surface of a resin film to plasma processing and then chemically adsorbing fluorine compounds thereto.
(2) JP H11-286784A discloses a method of applying a treatment solution including polycondensates of metal alkoxides, minute metal oxide particles, and silane compounds having fluoroalkyl groups onto a glass surface and drying the treatment solution, thereby forming minute recesses and projections in its surface.
(3) JP 2000-144116A discloses a hydrophobic film formed by applying an application solution including tri-alkoxysilane polycondensates onto a substrate and then thermally processing the solution to form recesses and projections in its surface.
(4) JP 2001-17907A discloses a method of forming a film by applying a solution including an aluminum compound onto a substrate and then immersing the two in warm water to form fine recesses and projections on its surface.
(5) JP 2001-207123A discloses a method of applying a solution, in which metal alkoxides and substances that separate from the metal alkoxides in solvent and that have the characteristic of degrading, combusting, and sublimating from room temperature up to 700° C. are added to a solvent, to a substrate and performing thermal processing to form a fine porous layer with an average pore diameter of 100 nm to 2 μm.
However, with the methods disclosed above in (1), (2), and (5), the film thickness and/or the recesses and projections of the films are large. For this reason, transmitted light is scattered, which raises the haze value (ratio), and thus the film has low transparency.
Also, with the technologies disclosed in (3) and (5), thermal processing at elevated temperatures is necessary after the application solution has been applied to the substrate. Thus, the substrate is limited to materials with high thermal resistance. Thermal processing is necessary.
With the method disclosed in (1), the recesses and projections are formed through plasma processing, and thus a processing device for this is required.
With the method disclosed in (4), the recesses and projections are formed through warm water immersion, and thus a device for supplying warm water is necessary.
Moreover, with the technologies disclosed in (2), (3), (4), and (5), a wet application film is formed as a dip coating, for example, and then dried, and thus variations tend to occur in glass end portions and in the film surface, deteriorating the outward appearance of the film.
Furthermore, with the technologies disclosed in (1), (2), (3), (4), and (5), the recesses and projections of the film surface have an irregular fractal structure, and thus the film has poor abrasion resistance.
For example, in a hydrophobic substrate, the contact angle of water increases as more unevenness is formed in the surface to increase the roughness of the surface. When this angle of contact exceeds 150° the substrate exhibits super hydrophobicity sufficient to make it difficult for water droplets to remain on its surface. To exhibit this degree of super hydrophobicity, a shape that allows a large amount of air to be held between the surface recesses and projections and the water droplets is considered necessary.
However, light is scattered when the surface has large projections of several hundred nm or more, for example, and in the case of a transparent substrate, this generates haze and results in a loss of transparency.
Also, with conventional super hydrophobic surfaces, the projections of the surface were formed in complex fractal shapes. For this reason, there was the problem that the ability to repel water was quickly lost due to abrasion, for example.